Insulin binds to insulin receptor on tissue
cells and tells them to absorb glucose. That
results in a decrease in blood glucose levels.

When you have not eaten for an extended period
of time, your blood glucose level drops and that
triggers the pancreas to release a hormone called
glucagon. Glucagon signals to the liver to
produce glucose from stored glycogen and
release it into the blood. The result is an
increase in blood sugar.

The balanced release of insulin and glucagon
is an import system to maintain homeostasis.
When this system is disrupted, people develop
diabetes. Interestingly, some animals have
dysregulated glucose homeostasis as a part of
their normal physiology. I just published a paper
describing blood glucose regulation in the Mexican
cavefish. Populations of this fish that evolved in
caves are unable to regulate blood glucose, yet
they are healthy. You can use this link to check
it out! cavefish
blood regulation

I could just say "all of them" and it would
not be far from the truth. The skin keeps
fluids in and the air out. The respiratory
system helps get the elements out of the air
that are needed, as well as expels some waste
products. The digestive system brings other
materials not part of the air in, and excretes
unwanted detritus. The excretory system
removes more waste products. All of it is
coordinated by the nervous system.The
skeletal and muscular systems enable movement
in order to get necessary materials. Really the
only system that I can't think of in maintaining
homeostasis in some way is the reproductive
system.